I think there are no definitive tactics, at most you need to know what works against what and how you can coordinate an attack, the rest is pure experience.
For example, you need to know how to smoke a zone, and how to micro it, but there isn't definitive tactics on how you can do an attack, you just need to know that the smoke is good to neutralize some defense platforms (ATGM's)
You should know that ATGM's are used for defence, and most vehicles that have them play a role in line defences, just compare the stats between a T72 Jaguar that has not ATGM's, and a equivalent Russian tank with atgm's, you will see how the T72 Jaguar is more fit for shock roles and the other one for defence roles (since the ATGM's have a horrible accuracy, you should amass them, and a shell is faster than a missile =P.). And smoke is exactly for that, to inutilize some of the ATGM's to overpass a zone. Though you can do this in a lot of ways, with Rocket artillery to shock their units, or with a pincer approach to confuse.
You should know how the morale affects the troops, a tank in shaken status will not fire accurately his gun, you should see what units are shaken and support or retire them between skirmishes or you could also see if the enemy has shaken troops or you could do something to make them lower their morale (For example, a way is massing ZSU Afghanskii because if they fire their autocannons at a target, even if they don't penetrate for shit, they will make shaken and after that you can push without problems).
But I think the most important thing in the game is to have the speed to deploy, if you deploy correctly in the first 2 minutes you will have much more advantage than the other guy (since you will have more reinforcements points per zone captured).
You should give a try the Category B decks with no nationality, since they have more troops per card you can literally a mass tanks and stuff with them (but use Cat B, since Cat C has too weak AA defense), and it will make your experience less frustrating since you will not lose the match if you lose a long-range AA (because you will have at least 3 more of them because of Category B).
Also, you should really plan your AA defences, since if you aren't protected against planes or helicopters you will have a bad time. You should have a first line defence with AAA guns and short-range IR missiles (like the Igla o Strela mounted on BRDM), to shaken ground and air helicopters. A second line defence with long-range IR seeking missiles (like the Tunguska with the AAA radar guns desactivated, or the OSA if you are using cat B decks). And a third line with long-range radar missiles with their radar desactivated (like the BUK), you need to have this deactivated because there are anti-radar missiles that will never miss, because they auto-detect radar missiles and are autolaunched from the planes (this is historical accurate, search for SEAD ARM planes), even a shaken plane will fire them and destroy your radar AA's, but if you desactivate their weapons on the HUD, they will be not detected as radars.
And the other thing, most guides show you that you should buy expensive t80 stuff and roll with them, but they are very bad advices to beginners, because first you will not know how to use them, sending them to be eaten by ATGM's. You should buy medium-class tanks (if they are Category B decks you will get more of them), and take Veterancy with them, why? well because with that a cheap tank like the T72 first edition will have more accuracy than the same point-tanks without veterancy, and since they use Kinetic rounds, at very low ranges even a T55 can penetrate a M1Abrams without problems.
Don't forget Recon, recon is the most useful thing in the game, since it gives info about everything that you can't see, you can plan what to do if you see it, like if you see a massed zone of AA you can plan doing a rush to try to kill them, or giving arty and then doing a raid, or whatever you think of.
When doing attacks, use all assets at disposition, pair the tanks in groups of two and not four (since when they are in group of four they fire at the same objetive the four tanks, that's highly ineffective). Plan your attack, see if the enemy has AA, wipe the AA (I think if you wipe the enemy AA you can win for sure), send smoke, send infantry to the woods and deploy them, send planes to napalm some forests near your troops, if you see high cost tanks, smile and send an MIG with Cluster bombs to take it (but first take the AA!, that's why it's important to recon), cover your tanks with choppers, and AMASS your attack in a flank, don't try to attack all the parts of the map at the same time, you will not have enough troops to make for it.
That was long, well, it's a good game, and you can learn a lot from it. When you know what thing is what and for what it's used, you will quickly improve. I suggest playing in 10vs10 matches since they tend to be less frustrating than losing in a 2vs2, but you should know that those 10vs10 also doesn't show what wargame is about.
PD: I suck at this game, those Tips were given to me from a guy that I know that is experienced on this game (he plays since European Escalation and participated in the Beta of Airland Battle), when I started and I followed them with good results.

Depends on the era, and your preferences, if you want something that needs thinking, something that needs reflexes, something that need planification, or something that is simply realistic.
Anyhow, for space, I would say Homeworld, it's simply great, very balanced and fun.
For Modern war, Wargame has an important point, though I have some complains towards it (the winning side gets more reinf points, so it becomes a uphill battle. However if it's pure fun until you get to that point :P), if you played games like World in Conflict or RUSE you will love this.
Total War games are very good, but they sometimes lack in the MP part (shogun 2 mp was very unbalanced, and if you didn't had the DLC's they battles were just painful to play). But I had the most fun with them, I had great times playing Europa Barbarorum online on 4 vs 4 matches using historical rules and stuff, they were really balanced battles and you need skill to win, I think you can still search for the community if you search on inet.
Warhammer is very fun too, I don't play it but i have friends that do and speak all the time about them lol
Also, if you want to have a definitive game and stay with it for a long time, you could consider start playing Go, Chess, or Shogi. I really mean it, if you can get started it's very possible that you will enjoy them for a long time.

As a side note, I tend to not compare Chess with Go directly, since it makes for some trolling sometimes :P
Anyway, here is another good page for people going into SDK ranks and dan players (Although DDK can use the page as inspiration maybe.): http://www.josekipedia.com/

I could suggest doing Life and Death problems, I got frozen up as 12 kyu for a long time, until I started doing L&D problems, and then got quickly to 9 kyu.
Most beginners tend to do life and death problems with the goproblems.com page, but I find it a little bad, since you don't really read the variations, you just click until you have the answer. If you do 15 or 10 L&D a day in the bus or in your free time (I do 30-25 easy ones), you will quickly improve, faster than with anything else at this point (mostly because beginners could have some stuff to think about concepts, but if you can't hold it up with good brute force, aka reading, you will get in weird situations a lot of times because you couldn't read it).
This is a must do for starting in life and death: http://tsumego.tasuki.org/ (See Elementary L&D of Cho Chikun)
Also I don't like too much the bots, since they are very weak at the opening stage, and sometimes they do very weird things, most players get some bad habits from them, you could try playing in a server like Tygem or KGS, and trying the fast games if they suit you (10 mins w/ 1 byomi 30 seconds or 3 byo yomi 30 seconds (blitz)), or slow games (1 hour/30mins, 3 byo yomi 30 secs)
See ya, and again, send me a pm if you want to improve, I can give you a few tips or review a few games

I think this is too relative, we are spoiled as old gamers as we already have some skills related to games (f.e. reflexes on fps), but there are still some games that need some care to win (see, Wargame, Total War series on MP, etc.)
And it's relative because I could say that Go or Chess are the most hardcore games and then all would be just lame to discuss, but it's not true, as every game need some set of skills, the thing is that modern games uses the same sets of skills of the rest of the games, so if you learn to play call of duty, you also learn to play battlefield at some point, or whatever-fps. For example, most people say that ArmA is the ultimate game in the difficulty aspect (i don't agree), but that's because you need a certain mentality when playing it (and also depends of the mission), rather than having a very high difficulty like for example Doom w/ Plutonia expansion :P.

Los Jaivas.
Sol y Lluvia
Those three are from a singer named Victor Jara
Here a rendition to Napolitan music (since they were exiled to Italia in the 73's with the dictatorship), the original one is known as "tarantella di Masaniello"